As condensation products of phenols and carbonyl compounds, bisphenols are starting substances or intermediate products for the production of a multiplicity of products. Of particular technical importance is the condensation product which results from the reaction between phenol and acetone, namely 2,2-bis(4-hydroxyphenyl)propane (i.e., bisphenol A, or “BPA”). BPA can be used as a starting substance for the production of diverse polymeric materials, such as, for example, polyarylates, polyetherimides and modified phenolformaldehyde resins. Preferred fields of application for BPA as a starting material include the production of epoxy resins and polycarbonates.
Methods for the synthesis of bisphenol A (BPA) by means of ion exchanger catalysis are disclosed, for example, in U.S. Pat. No. 4,391,997, U.S. Pat. No. 4,400,555, U.S. Pat. No. 4,590,303, and European Patent Application EP 0210366A. It is also known to produce BPA on a large scale by passing a mixture of phenol and acetone through a fixed bed reactor packed with polystyrene-based sulfonic acid ion exchanger resins and then working it up.
The reaction of phenol with acetone in the presence of acidic catalysts produces a product mixture (reaction mixture) that can contain, in addition to unreacted phenol and possibly acetone, primarily BPA and water. In addition, typical by-products (minor constituents) of the condensation reaction occur in small amounts, for example, 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane (i.e., o,p-BPA), substituted indanes, hydroxyphenylindanols, hydroxyphenylchromanes, substituted xanthenes and more highly condensed compounds containing three or more phenyl rings in the molecular structure. In addition, further minor constituents, such as anisole, mesityl oxide, mesitylene and diacetone alcohol, may form by self-condensation of the acetone and reaction with impurities in the raw materials.
Such by-products, as well as water, phenol and acetone, can adversely affect the suitability of BPA for the production of polymers and generally have to be removed by suitable processes. In particular, high purity requirements are imposed on the BPA raw materials used in the production of many polycarbonates.
Working-up and purification methods for BPA can be performed by removing BPA from the reaction mixture in the form of an approximately equimolar crystalline adduct containing phenol by cooling the reaction mixture and crystallizing out the BPA/phenol adduct in a suspension crystallization. The BPA/phenol adduct crystals can then be removed from the liquid phase by a suitable solid/liquid separation apparatus, such as a rotary filter or centrifuge, and purified further.
Adduct crystals obtained in this way can typically have a purity of more than 99% BPA relative to the minor constituents, with a phenol component of approximately 40%. Superficially adhering impurities can be removed from the adduct crystals by washing with suitable solutions that typically contain one or more constituents from the group comprising acetone, water, phenol, BPA and minor constituents.
The BPA/phenol adduct crystals obtained following the above-described suspension crystallization of the reaction solution and the solid/liquid separation are generally subjected to further purification steps that can include, for example, distillative, desorptive or extractive methods in which the removal of phenol and possibly the reduction of the concentration of minor constituents in BPA can be achieved.
Alternatively, the phenol can also be removed from the BPA/phenol adduct crystals by melting processes.
As mentioned, high requirements are imposed on the extraction of BPA/phenol adduct crystals from the product mixture. Therefore, in addition to the phenol present in excess, and possibly unreacted acetone, the process should also generally remove water and a multiplicity of other by-products of the bisphenol A production, such as, for example, the aforementioned 2-(4-hydroxyphenyl)-2-(2-hydroxyphenyl)propane, substituted indanes, hydroxyphenylindanols, hydroxyphenylchromanes, spirobisindanes, substituted indanols, substituted xanthenes and more highly condensed compounds containing three or more phenyl rings in the molecular structure.
Thus, for example, European Patent No. 1268379 B1 describes a process for producing BPA/phenol adduct crystals by crystallization. The production of BPA having a purity of 99.5% is described, preferably by a two-stage crystallization in which the product mixture is first cooled to around 50 to 70° C. and then to 40 to 50° C.
Since the purity requirements imposed on bisphenol A are always increasing, further process improvements are desired to ensure the increasing quality requirements, for example, for the production of polycarbonate.
This can be made possible, for example, by increasing the dwell time in the crystallizers. Thus, for example, it is known from European Patent Application No. 1607380 A1, that in the case of a short dwell time of 0.5 h or less in crystallization and correspondingly faster crystallization, the inclusion of reaction mixture or, in other words, the incorporation of impurities in the BPA/phenol adduct crystals result in a poorer bisphenol A quality, with the result that dwell times in the region of 1-3 h are preferred.
A further disadvantage of the method described in European Patent No. 1268379 B1 is that, in the case of such a two-stage crystallization, deposits may occur at the surfaces (fouling) in the circulation coolers of the 1st stage of the crystallization (high-temperature stage). To remove the deposits by heating to approximately 80° C., for example, the circulating cooler has to be shut down at regular intervals to remove the deposits by melting. During this time, the production has to be stopped or the crystallization has to be taken over by the second crystallization stage alone. This can result in production loss and/or reduced product quality characterized, for example, by an increased proportion of by-products (impurities) in the BPA.